The apparatus may be of a type for inspecting an object for example for inspecting its surface or, if the object is transparent to the radiation used in the inspection, for inspecting the bulk of the object. Preferred arrangements of the apparatus are particularly applicable to the inspection of sheet material.
Our previous patent No. 2 054 835 disclosed an inspection apparatus in which two beams of light which have been influenced by different portions of an object were passed to a grating 14, the two beams being spaced at the grating by an uneven number of widths "w" which correspond to the widths of the alternate light and dark areas of the grating. Light collected from the grating would be of a constant amplitude so long as the two beams are spaced apart by that exact distance, but faults or flaws in the object under test would cause the distance between the two beams to change and therefore cause a difference in the light received from the grating. This could be used to indicate a fault or flaw in the object or suface under test.
Although such a system operates satisfactorily there are a number of difficulties. Where a sheet material is to be inspected which is of considerable width, then a grating 14 of the same width as the sheet material must be provided. This increases the cost and complexity of the apparatus. Furthermore a cylindrical lens 26 is utilised and once again the cylindrical lens is an expensive component and the length must be sufficient so as to enable the whole of the width of the sheet material to be scanned.
Furthermore, the arrangement only deals with deflections of the beam in one plane so that deflections in the plane of FIG. 3 are detected, but deflections at right angles to the plane of FIG. 3 would not be detected and certain defects would therefore not be detected.
Another inspection apparatus is described in U.S. Pat. No. 3,790,287. Somewhat similar problems arise with this arrangement. There is described an apparatus for examining a silicon wafer which of course is of limited size. Thus the scanned beam passing to the wafer 28 passes through a lens system 26 which must clearly be of sufficient lateral extent so as to be able to receive the beam at the extremes of its scanning path. Clearly such an arrangement is limited to examination of quite small objects and would not be suitable for large sheets.
Furthermore, the arrangement only looks at faults on the surface of the wafer because an image of the surface itself is formed at the detector 38 (this is not totally clear but would seem likely from the ray diagram in FIG. 1) and so the arrangement is not suitable for examining, for example, transparent objects in which the beam passes through the object.